I dont like looking at walkthrough videos but i had no choice , if you look at the video at 4.56 you will see somebody about to place a cog into a box and you will see a gate open , he then goes round the corner and goes through the second gate. My problem is that the second gate is not open when i go to it. I have watched the same bit of video on 4 different walkthroughs and they all do the same moves as i have done and the gate opens.
View: https://www.youtube.com/watch?v=bKiz2InFzaM